1. Field of the Invention
The present invention relates generally to radio frequency distance measuring systems and methods, and more particularly to a method and technique for accurately locating the distance or position of an unknown point with respect to known point(s).
It relates to systems and methods using known reference point trnsmitters at known locations, also known as marker beacon systems, to determine the position of an unknown receiving point.
The present invention is of general utility in geodetic surveys, off shore navigation and exploration, air navigation, satellite positioning, vehicle location and general surveying work as used in the construction of highways, harbors and other civil engineering works.
2. Description of the Prior Art
Many phase or time comparison systems have been developed, and are in use, for distance measuring or positioning by radio means.
Among the so called Range/Range positioning systems, where two distances or ranges are measured from an unknown station to known reference stations (by measuring the velocity of light or radio propagation) are the systems disclosed in the following references:
U.S. Pat. No. 3,325,811 to Earp PA1 U.S. Pat. No. 3,397,400 to Maass et al PA1 U.S. Pat. No. 3,613,095 to Elwood PA1 U.S. Pat. No. 3,816,832 to Elwood PA1 U.S. Pat. No. 3,916,410 to Elwood.
Also, the well known OMEGA navigation system, in world wide use, is a conventional system.
In all phase or time measurement systems, stable frequency standards are used. The accuracy of measurements is directly related to the short and long term stability of the frequency standard sources. Two or more sources are brought together, either physically at the same point or to a known distance calibration point, and are calibrated together so that they are at the same frequency and phase. After these sources are separated, the accuracy of the system depends on the long term drift of the two or more sources. All radiated frequencies at the reference point are derived from the reference point source.
Some systems transmit one frequency only, (the OMEGA system), or some transmit a carrier and modulating frequencies (side bands). Conventional single frequency systems suffer from the disadvantage that the maximum distance that can be measured without ambiguity (without having measurements repeat in error) is one wave length. In the OMEGA system to obtain large unambiguous distance measurements, the operating frequency is low, typically 10 to 12 Kilohertz.
To be able to utilize any arbitrary, selected, distance measuring frequency without the need to radiate a cumbersome or awkward low frequency, systems were invented where a radio frequency carrier is pulsed, interrupted, or modulated at a given rate where the rate corresponds to a time period frequency used in the distance measurement. The carrier is only a carrier for the second time period information signal, though in the Elwood system the carrier may also be used to perform a phase measurement for one wave length and a small distance, with a greater distance marker available from the modulation, pulsing or bursting of the carrier.
All carrier modulation, interruption, pulsing or burst schemes suffer from the disadvantage that a second timing signal must be obtained, phase coherent with the reference frequency for generating the carrier, and used to modulate the carrier. Any form of modulation used, FM, AM, single sideband AM, double sideband suppressed carrier, interruption of the carrier or bursting of the carrier (100% AM) suffer from two major inconveniences. First, the modulator can introduce modulation errors in linearity, jitter, noise and phase shifting of the carrier due to the modulation circuits. Second, the modulation process itself of necessity generates side bands, thereby occupying more frequency space than needed, and requiring wide band receivers for reception.
The new and novel two continuous wave carrier signal system and method of the present invention is superior to those conventional systems described above, because while retaining the simplicity of a pure continuous wave radiation system, it allows the flexibility of a modulation system in radiating a second reference time signal without the use of modulation. The second time reference signal in the present invention is the difference in frequency between the two continuous wave carrier signals radiated from the same known point. More than two continuous wave carrier signals can be radiated from each reference or known point, and the present invention as described is not necessarily limited to two continuous wave carrier signals from each known point. The expression "two frequency system" will continue to be used throughout this document for simplicity.